1. Field of the Invention
This invention relates to a gas sensor and, more particularly, to an NOx sensor for sensing the concentration of nitrogen oxides contained in combustion gas or in exhaust gas from an internal combustion engine. The invention further relates to a solid-state sensor for sensing NOx gases, the sensor being suited to the sensing of NOx exhausted from combustion devices in general and NOx contained in automotive exhaust gas subjected to high temperatures in particular.
2. Description of the Related Art
An NOx sensor typical of the solid-state type disclosed thus far is described in the specification of Japanese Patent Laid-Open Publication No. 4-142455. This sensor includes a nitrate electrode and a reference electrode provided on an ion conductor residing in the sensed environment and measures an electromotive force produced across the electrodes. Though this sensor is sensitive to both NO and NO.sub.2, the sensitivities to NO and NO.sub.2 differ. As a consequence, the concentration of total NOx cannot be detected in a measurement environment in which both gases coexist and it is not possible to detect the concentration of NO or NO.sub.2 separately.
In an effort to improve the sensitivities to NO and NO.sub.2, there has been disclosed an emf-type sensor in which an auxiliary electrode is coated or mixed with an NO oxidizing catalyst. (See the specification of Japanese Patent Laid-Open Publication No. 6-123726.) In accordance with this proposal, NO contained in a gas in which NO and NO.sub.2 coexist is oxidized to NO.sub.2 so that a single component gas can be obtained. This makes it possible to detect the total NOx concentration. However, the accuracy of this method is decided by the oxidizing ability of the catalyst, just as with the conventional method of analysis, and a value different from the actual NOx concentration may be obtained. Further, since sensors of this type use a nitrate for the auxiliary electrode, problems arise in terms of resistance to humidity and heat. Difficulties in terms of long-term stability make it nearly impossible to put these sensors to practical use.
A sensor utilizing the semiconductor properties of various oxides to measure a change in electrical conductivity based upon the concentration of NOx has also been reported. However, since the sensitivity to NO and the sensitivity to NO.sub.2 differ in this sensor as well, the concentration of NOx in a measurement environment in which NO and NOx coexist cannot be sensed.
A recently proposed method electrolyzes NOx gas electrochemically and senses NOx concentration based upon the value of electrolytic oxygen ion current. (See SAE Technical Paper 960334 or the specification of Japanese Patent Laid-Open Publication No. 8-271476.) The detection principle of this sensor is itself based upon that of electrolytic current-type sensors used widely heretofore to sense other gases.
Specifically, this sensor has an ion conductor provided internally with two chambers. In the first chamber oxygen is drawn out by an oxygen pump to make the concentration of oxygen in the measurement environment substantially zero and reduce NO.sub.2 to NO. A voltage is applied to electrodes provided in the second chamber to ionize oxygen produced by the reduction to NO in the measurement environment. The resulting electrolytic current is then detected to sense the concentration of NOx. The NOx concentration sensed by this sensor varies greatly depending upon the performance of the oxygen pump. Further, in a case where the concentration of gas to be sensed is low, the concentration of residual oxygen in the measurement environment interferes with measurement. Moreover, since the signal current is extremely small, the S/N ratio is degraded in a noisy environment, such as in an automobile. This makes it difficult to detect NOx concentration accurately.
The inventors have proposed an emf-type NOx sensor and filed for patents. Though these proposals provide good sensitivity to NO or NO.sub.2 gas, there are instances where the NO and NO.sub.2 gases interfere with each other or are susceptible to interference from the reducing gas.
The inventors have further proposed (Japanese Patent Application No. 8-85419) a sensor not susceptible to interference from a reducing gas. This sensor includes an oxygen pump and an NOx sensing electrode formed on a solid electrolyte. When a reducing gas is oxidized, Nox gas is oxidized to NO.sub.2 gas at the same time, thereby suppressing interference. However, this arrangement does not necessarily provide a solution to the problem of mutual interference between the NO and NO.sub.2.
A noble-metal electrode is expected to serve as an excellent sensing electrode because it exhibits satisfactory resistance to heat even in a high-temperature environment such as automotive exhaust gas. In this respect a platinum sensor is in use as a .lambda. sensor or air-fuel ratio sensor in automotive vehicles and has demonstrated high reliability in actual use. Other advantages expected of noble-metal electrodes are chemical stability, ease of manufacture and low impedance. Examples of NOx gas sensors using a noble-metal sensing electrode on a solid electrolytic substrate of zirconia are referred to in the specification of Japanese Patent Laid-Open Publication No. 8-271476. These will now be described.
The first is set forth in the specification of U.S. Pat. No. 4,199,425. This specification discloses a sensor obtained by providing an automotive oxygen sensor (.lambda. sensor) of the concentration cell type with an alumina overcoat layer impregnated with rhodium in order to furnish NOx sensitivity. However, it is obvious that the rhodium-impregnated overcoat layer in this structure functions as an NOx decomposing catalyst layer and that oxygen produced by the decomposition of NOx is itself sensed by a platinum sensing electrode.
The second example is disclosed in the specification of Japanese Patent Laid-Open Publication No. 59-91358. This sensor includes a solid electrolytic substrate of zirconia, an electrode comprising a noble metal such as platinum, rhodium, palladium or gold formed on the substrate, and a sensing electrode formed on the substrate and obtained by building up or supporting an N.sub.2 O decomposing catalyst such as CO.sub.3 O.sub.4 on the electrode. A potential difference across these electrodes is measured. When measurement of NOx in automotive exhaust gas is considered, the gases of interest are NO and NO.sub.2 and measurement of N.sub.2 O is not performed Furthermore, the potential difference with respect to gases of low concentration is extremely low and there is almost no potential difference in the intermediate concentration region (less than several thousand ppm) of actual exhaust gas.
Thus, even if a sensing electrode made of noble metal is used in a concentration cell type NOx sensor according to the prior art, the function of the electrode is merely that of an NOx decomposing catalyst and the electrode merely acts as a collector that collects electric charge involved in the electrolytic reaction with the catalyst layer. Further, as set forth also in the specification of Japanese Patent Laid-Open Publication No. 8-271476, the conventional NOx sensor using a sensing electrode of noble metal develops only a small potential and is highly dependent upon the concentration of oxygen in the environment of the sensed gas. The state of the art is such that these sensors can be applied only in a direction that decomposes NOx.
The electrode potential of a sensing electrode decided by NOx and O.sub.2, namely the nitrogen-oxide sensitivity of a mixed potential-type NOx sensor that outputs an electromotive force with respect to a counter electrode, is influenced by the conversion efficiency of a gas equilibrium reaction between NO and NO.sub.2 and the conversion efficiency of the electrode reaction, as a result of which the output signal of the sensor electrode is unsatisfactory. Accordingly, a sensor having a higher sensitivity is sought. When emf is detected, the reference electrode potential varies greatly depending upon the type of gas that takes parts in the electrode reaction. In addition, the concentration of the gas that takes part in the electrode reaction has a major influence upon the emf of the sensor electrode. As is well known, the equilibrium between NO and NO.sub.2 shifts in the direction of NO as temperature rises, and NO.sub.2 obtained by a change brought on by the electrode reaction decomposes into NO. This causes a decline in emf when NO.sub.2 is sensed. However, if nitrogen oxide in a gas to be sensed is oxidized to a peroxide of nitrogen of order greater than NO.sub.2, the standard equilibrium potential of the peroxidized nitrogen oxide rises and it is possible to achieve a sensitivity in excess of the emf obtained with NO.sub.2 gas. Furthermore, if the concentration of oxygen in the environment surrounding the sensing electrode is raised, advantages are gained in terms of producing the peroxidized nitrogen oxide and it is believed that the reaction through which NO.sub.2 is decomposed into NO can be suppressed.